Frequency hopping networks are widely used in wireless communications. For example, frequency hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both the transmitter and receiver. It is typically used as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA) scheme.
A frequency hopping network offers three main advantages over a fixed-frequency network. First, signals in a frequency hopping network are typically highly resistant to narrowband interference, for example. Signals in a frequency hopping network may also be difficult to intercept. For example, an FHSS signal typically appears as an increase in background noise to a narrowband receiver. Signals in a frequency hopping network may also share a frequency band with many types of conventional transmissions with reduced interference.
Because of the changing frequencies in a frequency hopping network, most frequency hopping networks cannot support the time division multiple access (TDMA) scheme. This is because it is often relatively difficult to control which communications device in the network is allocated time.
One approach to address the timing in a frequency hopping TDMA (FH-TDMA) network includes using fixed frequency rules, where each communications device is informed of timing information using a single frequency, for example. More particularly, a static time server, a beacon, or a rendezvous channel may be used. For example, a given communications device may be pre-designated as the static time server. However, a failure or stoppage of the communications device designated as the static time server would cause a failure of the entire network.
Additional communications devices may be designated as a static time server so the network does not have a single point of failure. However, coordinating or selecting the additional static time servers may be relatively complex. For example, conflicts may arise when two or more static time servers are selected. Conflicts may also arise as static time servers move in and out of communications range with other communications devices and each other.
U.S. Pat. No. 7,710,944 to Yoon et al. discloses a method of time of day synchronization between network nodes. More particularly, a network that includes a plurality of nodes is arranged in islands. Head nodes from each island are in communication with each other. Network time is synchronized with a node that has local global positioning system (GPS) time. Network nodes transition to common GPS time after an island or group head node determines that the transition in network time does not disturb the communication links.